Although many interest groups work together perpetually, most academic studies agree that coalition formation does not lead to more influence. In this article, we try to explain these puzzling findings. While former research generally tends to frame the decision of forming an interest group coalition as a strength, in this paper, we argue that coalition building should be considered as a ‘weapon of the weak’. Interest groups fearing that they are insufficiently influential, and whose very existence as an organisation is at risk, are more likely to coalesce. This theoretical framework is tested on a sample of around 3000 interest groups in six European countries – Belgium, Italy, Lithuania, the Netherlands, Slovenia and Sweden – and the European Union. Empirical findings clearly demonstrate that perceived fears – oriented towards both organisational survival and policy influence – have an effect on how likely it is that an interest group will decide to build a coalition.